by 1943 they lost 6 million out of 34 million mobilised , there was plenty of slack left.
The Soviets did mobilise 34.5 million men and women for the armed forces,
the formations coming under other departments (like the NKVD) and industry, of which 4.8 million, presumably those with specialist skills as well as those with limited fitness, were sent disproportionately to the latter two categories (Krivosheev p91). That leaves, rounded up, 30 million that could be easily sent to the front against Germany.
By the launch of Operation Uranus, the Soviets had suffered slightly under 6.2 million (p104) dead and captured (including DoW). They had also suffered a large number of permanently disabled, 11.5% of the sick and wounded, according to Krivosheev (p91). That's slightly under 0.6 million by 18.11.42. Lastly, during the retreat of 1941, the Soviets lost 0.5 million reservists called up but captured by the Germans before they could be taken on strength by the Red Army. In sum, the Soviets had lost about 7.2 million men by Uranus.
By then, they also had about 10.6 million men on active duty and somewhat more than 0.8 million in hospitals, for a total of about 11.5 million men in the armed forces (
Soviet fronts strengths: Overview). This would indicate of total consumption of about 18.7 million men on a total of about 30, thus leaving roughly 11.3 million more men for the rest of the war, right?
Wrong. According to this
post by Art, the Soviets had also lost access to over 5.6 million men up to the class of 1924 (mobilisable in 1942) in the conquered territories by the time
Fall Blau stalled in September. Arguably some of those were killed by the Germans and others were sent to work in Germany, but then again the classes of 1925, 1926 and 1927, all mobilisable before the end of the war, are not counted in the 5.6 million. So let's say the Soviets could gain from reconquered territories during the war 4 - 5 million men. Our pool of "used resources" now totals about 23 million men. That leaves roughly 7 million for the continuation of the war, of which (according to Tschadenko) about 0.9 million were fully fit and immediately mobilisable, 0.7 million (class of 1925) were mobilisable in 1943, a significant number were in the classes of 1926 - 27 and couldn't be immediately mobilised and the rest, perhaps 3 million men, would have to be combed out from men overaged or with limited fitness, the industry, the penitentiary system or the security organs. That would give, on average, about 2.5 million new recruits each year for 1943 - 45, a number which squares quite well with the about 2 million men mobilised during the autumn of 1942 (decree 2100) and winter of 1943 (decree 2640) - but a number also largely insufficient to cover irrecoverable losses at the rate the Soviets had experienced them until then.